Lab-on-a-chip technology has made rapid progress for applications in cell biology and biochemical assay.1 Lab-on-a-chip systems that enable the efficient performance of assays with low reagent consumption typically contain features on the structured surfaces of the microfabricated devices such as microfluidic chips and microwell arrays where air bubbles can be easily trapped upon the addition of a solvent or solution. The trapped air bubbles result in Cassie-state wetting on the surface. This wetting phenomenon has been exploited for specific applications, such as selective deposition of proteins and cells to the areas that are in contact with the aqueous solution, for example, on the surface between microwells (but not inside microwells),7 or on the top surface of micropallets (but not in the space among micropallets).8 Nevertheless, for a majority of applications, the trapped air bubbles in microfabricated devices are an obstacle in the use of the device and the air bubbles need to be removed to allow the entire surface to be in full contact with solutions of analytes, cell-culture medium, or other fluids.9, 10 
Microwell arrays, useful platforms for cell culture and assays at single-cell resolution, are examples of microfabricated devices possessing gas-entrapping features2, 11 Since microwell arrays are often made from polymers, such as PDMS, which are either hydrophobic or only slightly hydrophilic in their native form, trapping of air bubbles inside the microwells are encountered whenever the array is covered with an aqueous solution. To solve this problem, plasma treatment is generally used to make the surface hydrophilic; however, in many of the common polymers this hydrophilization is only temporary, and either a partial or complete hydrophobic recovery is usually observed.14, 15 In addition to surface oxidation, several methods are currently used for removing trapped air bubbles in cavities, including vacuum application, pressurization, centrifugation, vibration and sonication.10, 16-18 Alternatively, low surface tension liquids (e.g. ethanol, γ=22.4 mN·m−1 can be used to initially wet the surface prior to exchange with water (γ=72.9 mN·m−1) or an aqueous buffer.19, 20 Besides microwell arrays, corners and dead ends in microfluidic channels have similar problems with surface wetting and bubble formation. A new microfluidic design, called a phaseguide, based on a step-wise advancement of the liquid-air interface using the meniscus pinning effect, can effectively eliminate the probability of trapping air bubbles in complex microfluidic geometries such as corners and deep angular structures.6 However, this method is difficult to remove trapped air in microcavities microwells, or dead ends, since it relies on the creation of strips of material on the wall along the direction of advancing fluid.
Although all of the above methods are effective in preventing or removing air bubbles in specific cases, there remains a need for a simpler, passive method for preventing the formation of gas bubbles or removing gas bubbles from microfabricated devices having microcavities, corners, dead ends and other gas-entrapping features.